Signals returning to danger

[londonstuff]

I remember reading about an incident in the 60s or 70s about an early morning reversing train going the wrong way near West Ken/Barons Court (although my memory may be playing tricks on me) and the train operator driving the 'right' train only knew something was wrong when a signal went from green to red.

Is there ever any 'legitimate' way of a signal that's green ever returning to danger, other than in failure conditions or being reset by the signallers if a wrong route has been set?

[mrfs42]

You'll find the account of the incident on the Picc on Tubeprune's website.

Signals will, of course be legitimately returned to red by the passage of trains, although there is a lot of attention paid (principally on NR) to whether or not these are 'first wheel replace' or 'last wheel replace' - clearly the last thing a signalling system needs is a Motorman seeing a signal dropping back to red legitimately and then putting the anchors on because the signal is now at red.

Other legitimate occasions where a signal can drop back without failure and route-changing is when a train protecting a converging junction has a train just nudges over the approach track circuits on the other line - the signalling system would 'see' this as a potential conflict and put all affected conflicting signals back to danger: 'flank protection'.

Likewise if a train overuns the normal stopping mark in a terminal platform and breaks what is known as a 'track circuit interruptor' signals will be held at red - because the system will 'think' that the train has derailed. However, this isn't quite what you are asking.

If for any reason the signal operator has information that needs the approaching train to be held, the route would be cancelled, returning the signal back - though this will not necessarily mean that the actual route set is changed - just the route 'called' by the signal.

I'm sure there are plenty of other examples too, but I can't immediately think of any others.

[Chris M]

One additional scenario that comes to mind is when an adjacent line needs to be protected. For example on a two track railway running north-south, if there is an incident on the northbound line (perhaps a northbound train has derailed foul of the southbound track) then signals on the southbound track ahead of the incident will be put back/maintained at danger. This could be done automatically (e.g. in the case of a serious incident breaking the track circuit on the southbound line) or manually by the signaller on being informed of the incident.

[Deleted]

or track circuit shorting clips